Last week, the Office for National Statistics published its latest population projections (pdf), which extrapolate from current trends to suggest the UK will grow from 62.3 million people today to 70 million in 16 years.

The ONS also estimates 47 per cent of this projected increase would come from immigration, and a further 21 per cent from the children of future migrants.

Today, reacting to these figures, Andrew Green and his pressure group MigrationWatch launched an e-petition, titled “No to 70 million”, to:

“…call on the government to take all necessary steps to get immigration down to a level that will stabilise our population as close to the present level as possible.”

The petition was given prominent backing by today’s Daily Mail, including an article by Green, and in the Sun, with an article by Nicholas Soames and Frank Field, the joint patrons of the “Balanced Migration” group, the parliamentary wing of MigrationWatch.

If we look at the petition and the supporting articles, there are a number of odd or problematic aspects to the way this campaign has been framed.

The odd thing is that, in contrast to other e-petitions which have hit the news, this one calls on the government to do something which the prime minister has already made a high-profile pledge to do. His recent speech contained a read-my-lips repeat of the pledge – although a poll in the Sun shortly before found 78% of people doubting he would deliver it.

The problems include the familiar refrain “we aren’t allowed to talk about immigration”, when often it seems we do little else (not least given the sterling efforts of the Mail and Sun), and the sturdy, un-killable fallacy of confusing net changes in employment with gross changes, with Andrew Green again complaining that while employment is up by two million in the last decade, the number of foreign-born workers is up by 1.9 million.

This argument that “90% of new jobs go to foreigners” has been systematically debunked by various people, including me, Jonathan Portes, and neatest of all Declan Gaffney, who, on these pages, showed that by the same methodology, it turns out to have been the disabled who took all the jobs.

The final problem is the reliability of the projections which have sparked this debate. The ONS itself is at pains to point out they simply extrapolate from recent trends, that “are not forecasts and do not attempt to predict the impact that future government policies, changing economic circumstances or other factors might have on demographic behaviour”.

Earlier projections for the UK population in the year 2000 varied from 55m in 1957, up to 76m in 1965, and then back down to 65m in 1970: the story goes that after one particularly alarming projection, the government set up a working group to plan how to respond to the increase – but by the time of its interim report, the projected increase had been cut in half, and by the time of its final report, it had disappeared.

But let us leave these odd and problematic aspects to one side, and take the “No to 70 million” campaign on its merits, focusing on the question of population growth, and the contribution which immigration makes to that.

The picture of Britain as a ‘crowded island’ was the animating idea behind the government’s choice of net inward migration for its key immigration target – a choice whose implications are starting to dawn on ministers, as migration from eastern Europe and changes in emigration trends push it further from their grasp.

It is true the idea of Britain as a ‘crowded island’ resonates with voters – but how accurate is it? In his article today, Andrew Green claims we are the fifth most crowded nation on earth, if we disregard “small islands and city states”. His article does not say where he gets this information from (nor, as far as I can see, does the MigrationWatch website), but the latest UN figures (for 2010) place Britain 39th out of 196 nations.

And while it is true that many of the higher places are filled with small islands and city states, there are also plenty of big islands, or archipelagos, like Taiwan, Sri Lanka and the Philippines; some big developing countries, including Bangladesh and India; and several developed countries, not only the two Green mentions – Netherlands and South Korea – but also Belgium and Japan, all of whom would still be more densely populated than us even if we hit 90 million.

We should think a bit harder before we decide it would it really be so bad, or unmanageable, if we carried on growing through migration at, say, 150,000 per year.

Over ten years, this would add a million and a half to our population, plus a bit more through the children of these migrants – though the fears of Green and others about accelerating population growth from migrant babies are likely to be misplaced, since throughout history, birth rates of migrants and their descendants have tended to converge on that of their host country, just as birth rates of country folk moving to the cities have tended to converge on their new neighbours.

Still, even if we treat current fertility trends as fixed, net migration of around 150,000 per year would lead to population growth of just over two million a decade.

This is far from trivial, and definitely needs proper planning, but it is worth putting it in its historical and geographical context: this is broadly in line with the rate of growth Britain experienced between 1900 and 1970, and well below the current global average – where the same UN figures put us around 140th. In particular it is far lower than most of the countries whose influence in the world is growing.

If you look at which countries actually achieve the alternative of zero or negative net migration, it is not exactly a list of countries feeling happy and confident about their position. The irony of last week’s debate, which included the observation that on current trends we will overtake Germany in around 30 years, is that the Germans are probably more worried about this than we are, for the opposite reason.

Of course, if we are to take seriously the talk about “crowding”, focusing on national populations and densities is misleading, since it is really London and the South East which is crowded, rather than our island as a whole. Some towns and cities, particularly in the north, are struggling with the opposite problem.

The fundamental driver is economics, rather than migration or birth rates, and the issues it raises – of planning for infrastructure, housing, and services – would remain even in the hypothetical scenario of zero net immigration. These issues are mentioned in Green’s article in the Mail, as well as in the coordinated article in the Sun by Soames and Field, with the usual focus on pinch points in transport, schools, and hospitals.

But the assertion that 70 million is somehow the limit of “sustainability” seems entirely arbitrary – nothing more than the usual lazy fixation on round numbers. And again, these issues of sustainability and crowding are not unique to Britain: in terms of the rate of growth of our urban population (using the same UN figures) Britain ranks around the same as it does for population growth as a whole, around 140th.

It is notable the e-petition, and the supporting articles by Green, and Field and Soames, all demand “urgent action” without specifying the detailed policies that would be required to achieve their real aim, of zero net immigration. Their support for the Conservatives’ pledge to reduce net immigration to “tens of thousands” is purely tactical, and is helped in part by its ambiguity – i.e., whether it means the 40,000 Green calls for, or the 95,000 which current briefings tend to assume.

Zero net migration would require us to leave the EU, stop British nationals marrying foreigners, and turn all working migrants into guest workers.

At the moment, the coalition is proposing only the third of these. I suspect that, for all people’s negative views about immigration, if presented with these actual policies, most would reject them.

Indeed, a recent survey which probed people’s views on immigration somewhat deeper than the usual questions, found only weak support for many of the restrictive policies the government is pursuing, even though they won’t get anywhere near zero migration. One of the interesting findings was that, among the 69 per cent who say immigration should be reduced, a majority say the reduction should come “only” or “mostly” from illegal immigration, not touching legal migrants at all.

The strongest argument which MigrationWatch, the Mail, the Sun and others have, is not that people aren’t allowed to talk about immigration, or that politicians don’t talk about it, but that there has been a shortage of debate about the separate but related issue of population – what an ideal level should be, how far government should try to interfere in that (with all the implications for family policy as well as migration), how we should plan to deal with it, and how that fits in with broader global issues of population growth, density, urbanisation, resource scarcity, emissions, and other related questions.

Now is a good time to have this debate, as we welcome the arrival of the seven billionth baby. But even in global terms we should keep things in context: it has been estimated the current global population could fit into the state of Texas, if it was as densely populated as New York City.

What we need above all is a serious debate, not apocalyptic scenarios to scratch the millenarian itch for a secular age. There are a number of more sensible e-petitions on the government’s website which call for such a debate, and languish forgotten with a few tens or hundreds of signatures – while the MigrationWatch petition, backed by the two biggest selling dailies, will surely reach the required target in quick time.

We should all prepare to make the best of the immigration-centric, UK-centric, round-number-centric debate that follows.